Greetings,

The w124, at least 90 - 93 have a diesel fuel pre-heater. It's also present on, at least some, of the w126 from the 80s. I do not recall it on my w123.

There seem to be two components. On the driver side of the block, below the IP is a thermostat. It diverts fuel that is below 25 C to a heat exchanger. If the fuel is already warmer than 25 C it is not heated. I have two questions -

1. I find the thermostat easily enough. Where is the heat exchanger?

2. Why have a thermostat? Why not always pre-heat the fuel?

Every time I post questions and receive numerous replies that are accurate, helpful and pleasant I again think what a great resource this place is.

I've posted some drawings of the components. They are for a 6 cylinder diesel but are still relevant.

The Good Doctor

------------------------

'90 300D (now missing the BFS)
'99 E430
'99 SLK230
'81 300D
'75 240D

Attached Files

	Fuel system w24300d.pdf (55.3 KB, 741 views)
	W124_300DFuel pre-heater and thermostat.pdf (50.3 KB, 841 views)

The coolant warmed heat exchanger is wrapped around the back side (towards the firewall) of the oil filter housing.

Cold fuel keeps other components in the fuel system cool during operation - primarily injectors.

Thanks. Then when the WVO/SVO/etc people talk about heating their fuel prior to injection is only done by heating the lines from the IP to the injectors?

I haven't done an oil change on this car. I'll take a look at the oil filter housing and see if I can find the unit. Thanks for the help.

Is actually inserted into the engine upper portion between cylinders 4 and 5.
It's a Parasitic exchange from the cooling system.
You won't "See" it unless you look underneath the Intake Manifold.

__________________
'84 300SD sold
124.128

1/ hidden-ish in the cabin heater line from the block.

2/ hot fuel is bad for an injection pump (lubricants change to cutting compounds with temp rise) and hot fuel expands but injection pumps always meter by volume so you lose fuel delivery volume proportional to the expansion of the fuel, which is not an insignificant amount.

it is NOT for cooling, the fuel flow amount is insignificant... just do the math, literally a handful of calories per injection volume.

Yup, mine is routed up to the back of the OM602 head somewhere.

__________________
1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

Main reason I understand for not heating diesel is because it kills the lubricity. Like w124 said. With wvo there is an abundance of lubricity so we bypass that thermostat and go through the heater all the time.

Vw tdis after 1998 have a fuel cooler on them on the return line before the tank. They don't have a fuel heater. The injection system gets the fuel pretty hot. I don't know why they thought they needed a fuel cooler though. There were lubricity scares and problems when going to ULSD here in the states though.

__________________
What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Same reason hydraulics have a cooler, common rail systems are no more than (very) high pressure hydraulic systems, maintaining that high pressure basically converts every calorie absorbed by the fuel / hydraulic pumps into heat energy in the fuel / fluid.

It takes a LOT of energy, and therefore heat input, to maintain a 2,000 bar (30,000 psi) fuel system pressure...

Pressure AND temperature do odd things to compounds, while it is true VO is more "lubricant" (in some ways) than #2 diesel, that doesn't actually mean a lot once you cross the transition threshold for that particular fluid, does this mean that the combination of increased viscosity and pre-heating pushed VO through that threshold?

I don't know.

I do know that this combination does what it does to every other lubricant fluid, it reduces lubricity.

I also know from running machine tools that hydrocarbon fuels and lubricants behave in predictable ways when used as coolants and cutting compounds on machine tools, and those ways are very predictable indeed when you start to push the parameters of feeds and speeds and often the behaviour of these fluids changing gives the best early warning that you are pushing too hard, before the work suffers or you break a tool.

I have used VO as a cutting compound, various varieties, both straight and emulsified, it was bloody useless because it was totally unpredictable and suddenly went from being beneficial to being a part of the problem and ****ing up both the work and the cutting tool, after a week of experimentation I gave up and went back to the commercial fluids.

The fuel heater is a heat exchanger that uses coolant on its way to the cabin heater core; it’s basically a pipe with a loop of fuel line married to it within a stamped metal shell. The heat exchanger attaches to an outlet in the cylinder head by one O-ringed end inside the outlet and a clip secures it in place, the other end is a hose bib fitting onto which a hose from the heater core through the firewall attaches.

The first pic shows one in place on a 602 engine all 60X engines which have the fuel pre-heater have it in this position. The red arrow points to the heat exchanger itself and the chartreuse arrow to the cylinder head outlet connection



The next two show a heat exchanger removed from a vehicle.





I suspect that the primary reason for this system is not to heat the fuel for combustion efficiency reasons as much as for fuel filtration efficiency reasons, although that might be a side benefit. A common occurrence in diesel installations is under cold conditions for fuel to gel or wax-up causing filter clogging; this system heats the fuel prior to filtering. The fuel would under normal operating conditions be heated some as it passes through the IP which is lubed by operating temp crankcase oil and is at least at engine operating temps after a few minutes, the injectors are at least at engine operating temps then also.

OK. I'll go look for the exchanger. Just to satisfy my curiosity.

The VO people I encounter stress the value of pre-heating so I was curious.

Regarding the decrease in fluid volume with thermal expansion I'd think that would be modest and easily compensated.

The change in the properties of the fluid is intriguing yet the oil in the case is much hotter so, of course, some fluids are OK at higher temps.

I was thinking about bypassing the thermostat and always heating but given these comments I wont.

Take care.

0.046% per degF for #2, so 150 degrees F of preheat = 6.9%

Not what I would call "modest"...

Since you run dino, here's a good read
http://www.chevron.com/products/prodserv/fuels/documents/Diesel_Fuel_Tech_Review.pdf
above link is buggered, use this one
http://docs.google.com/viewer?a=v&q=cache:URQ-yk9LCFUJ:www.chevron.com/products/prodserv/fuels/documents/Diesel_Fuel_Tech_Review.pdf

I agree it is not modest. I had not expected that much expansion.

Wouldn't the ALDA does not have enough have enough variance to be able to correct for this?

The question is purely academic as I am not doing this.

I appreciate the links to the pdf files and will read them shortly.

Thanks.

Nice article of diesel fuels. I'm trying to figure how to download it because I would like to save it.

I'll probably send an e-mail to Chevron.

I notice on page 43 Chevron states that the industry does not encourage bio-diesel/petro-diesel ratios to exceed 5% because of lack of evidence of long-term compatibility.

Bio-diesel is much closer to petro than WVO so the industry view of WVO/petro blends would probably be at an even lower blend.

I'm not going to get involved in the whole VO thing again. Just pointing out that conventional business does not have sufficient data to go forward.

To suggest that they do this to protect their profits from petro I don't believe will stand up to careful thought. Raw oil from the ground is used in thousands of products. The oil companies would still be selling everything they could pump even if VO becomes attractive.

If the last paragraph causes someone to get upset then just ignore it.

TGD